1. Field of Invention
The present invention relates to a bearing, particularly a Cu-Pb series bearing, and a method for producing the same.
2. Description of Related Arts.
In Japanese Examined Patent Publication No. 39-22498, there is a proposal for producing an In-containing Cu-Pb series kelmet bearing. According to this proposal, rolled steel is subjected to coating of molten Cu to prepare a long strip-form substrate having a surface layer of Cu alloy. This Cu plating layer is subjected to plating of molten Pb-In alloy. The so obtained bearing has a Cu alloy layer, an intermediate Cu-In alloy layer formed due to diffusion of In into the Cu alloy, and a solidified layer of Pb-In pool.
It is known from Japanese Examined Patent Publication No. 46-3654 to include Te besides In into the Cu-Pb series kelmet bearing.
The other method employed for producing the kelmet bearing than the method described above is a sintering method. In the sintering method, Cu-Pb powdered alloy or Cu metal powder and Pb metal powder are dispersed on a hoop used as the substrate, and the hoop is passed through a sintering furnace and heated. The bimetal bearing is thus obtained. There is also a combined, sintering and hot-dip method, in which the bimetal bearing obtained by the sintering method is passed through a Pb-Sn bath to replace the Pb of bimetal with Sn in the bath.
The kelmet bearing is provided with a soft alloy, such as Pb-In alloy on the kelmet alloy-layer via a Ni barrier, except cases where the kelmet bearing is used under a low load.
Recently, bearing troubles arose in Diesel automobiles using sintered alloy-series kelmet bearings, when the running distance amounted to 100,000 km to 150,000 km. The present inventors investigated the bearings which caused trouble and confirmed the following. That is, in such bearings, the overlay was locally scratched off, although this local scratching did not lead to seizure. Occasionally, local corrosion or wear of overlay occurred to partly expose the kelmet, and Pb of the kelmet was removed from an inner part of the overlay through the exposed surface of the kelmet. Also, occasionally, cracks were generated in the overlay, and Pb was dissolved and flowed out through the cracks, thereby causing the disappearance of Pb from inner part of kelmet. The present inventors compared the Pb-disappearance degree and situation with the troubles of bearing and then could determine casual relations between them. In addition to the investigation of the bearings used in actual engines, the present inventors carried out investigation to reproduce in the laboratory the Pb disappearance from the kelmet surface and to compare the Pb-disappearance degree and situation with the performances of bearings. The conclusion arrived as a result of the investigations was that, in order to ensure the performances of bearings of Diesel engines for a long running period as described above, the Pb disappearance needs to be prevented or suppressed.
The above described trouble of bearings do not occur in gasoline engines during a long distance running. It is however anticipated that similar trouble may occur even at a short running distance, when the power and rotation of gasoline engines are enhanced. The countermeasure against the Pb disappearance seems therefore to be useful for any troubles of bearings used in gasoline engines. The situation of Pb disappearance is described in more detail with reference to FIG. 2.